The inventors came across the problems associated with apparatus of this type in the field of aviation. In aircraft, where mechanical, and above all thermal, constraints are significant, it is essential to ensure that the system for ventilating electrical or electronic equipment is reliable so as to ensure that the equipment will be cooled under all circumstances. When the equipment constitutes electronic circuits for automatic pilot control (or any other vital system), it will readily be understood that faulty ventilation, which would inevitably lead to the circuits overheating, could have consequences that are disastrous for the safety of the aircraft.
The European patent application published under the No. EP-0 588 414 discloses a forced air cooling system for electronic equipment, the system comprising:                an enclosure surrounding the components of the electronic equipment, said enclosure defining a path for the flow of cooling air;        a first blower mounted on the air flow path so that practically all of said flow passes through the first blower;        a second blower also mounted on the air flow path so that practically all of the air flow passes through the second blower;        breakdown detector means; and        switch means responding to the breakdown detector means to switch one of the blowers from an out-of-service mode to an in-service mode so as to compensate for the effect of the breakdown.        
That type of system gives satisfaction in the sense that it enables a breakdown, to be taken into account, and in spite of the breakdown it enables cooling of the electronic equipment to be maintained. Nevertheless, when a breakdown occurs, the blower that has broken down then constitutes an obstacle to air flow, since as mentioned above, practically all of the air flow passes through the blower.